In oil and gas wells where the casing is subject to movement down hole, failures can occur in the casing or in the casing connection. The movement of the casing can be caused by many factors such as shifting formations, formation pressures, overburden pressures, and thermal expansion and contraction from steam injection operations. Stresses induced to the casing from factors such as these, can buckle the casing wall, or cause connections to part or leak.
Horizontal well drilling is increasingly becoming a popular method of producing oil and gas from formations. Some of these horizontal wells have a shallow vertical depth, and require large degree build angles to hit the target. While existing casing connections can be designed to handle predicted changes in trajectory as the wellbore shifts from the vertical section to horizontal, such as for example bends 7° or less, unexpected doglegs may present larger build angles, as high as 30°, that are harder accommodate without failure or leaking.
Doglegs are defined as particularly crooked places in a wellbore where the trajectory of the wellbore changes more rapidly than anticipated or desired. A dogleg is typically described in terms of the number of degrees of trajectory change per 100 feet/30 meters. Doglegs often occur when characteristics of the formation do not allow for or support drilling at the desired build angle to create the horizontal wellbore. In such cases, large degree bends may be necessary to return drilling trajectory to the desired angle. Such large degree bends are often not tolerable by existing casing connections.
When there is a casing failure in a well bore, often, the well is lost. When running a rigid casing string through a build section of a horizontal well, often the casing is unable to pass through, or casing connections are damaged due to bending.
All casing connections can withstand bending to some degree, but in most cases, the connection will leak or part when bent. Most connections rely on the threads to deliver a seal, as well as the torque and tensile strength of the connection. Once the thread has a bending load applied to it, the integrity of the connection is drastically reduced or lost completely.
In designing casing connections a number of sizing requirements must be met that also pose a challenge to providing flexibility. Firstly, the annular allowance between a drilled wellbore and the casing to be installed the wellbore is very small, often in the order of ½″ to ¾″. This means that the outer diameter (OD) of any casing connection must match as closely as possible the OD of the casing to be connected. Similarly, there is a need to maintain a minimum inside diameter (ID) of the casing connection that matches as closely as possible the ID of the casing to be connected, so that tools maybe deployed in the casing without the formation of bottlenecks. Furthermore, casing connections must meet wellbore equipment minimum strength and durability requirements, while still being able to provide flexibility.
It is therefore desirable to develop a casing connection that can allow bending and rotation of the casing during installation and operation. It is further desirable to provide flexibility to standard, stiff casing so that it can be deployed in more severe doglegs, or bends, without departing greatly from ID, OD and strength requirements of the casing sections to be connected.